Revertant mosaicism for family mutations is not observed in BRCA1/2 phenocopies

Abstract

In BRCA1/2 families, early-onset breast cancer (BrCa) cases may be also observed among non-carrier relatives. These women are considered phenocopies and raise difficult counselling issues concerning the selection of the index case and the residual risks estimate in negative family members. Few studies investigated the presence of potential genetic susceptibility factors in phenocopies, mainly focussing on BrCa-associated single-nucleotide polymorphisms. We hypothesized that, as for other Mendelian diseases, a revertant somatic mosaicism, resulting from spontaneous correction of a pathogenic mutation, might occur also in BRCA pedigrees. A putative low-level mosaicism in phenocopies, which has never been investigated, might be the causal factor undetected by standard diagnostic testing. We selected 16 non-carriers BrCa-affected from 15 BRCA1/2 families, and investigated the presence of mosaicism through MALDI-TOF mass spectrometry. The analyses were performed on available tumour samples (7 cases), blood leukocytes, buccal mucosa and urine samples (2 cases) or on blood only (7 cases). In one family (n.8), real-time PCR was also performed to analyse the phenocopy and her healthy parents. On the 16 phenocopies we did not detect the family mutations neither in the tumour, expected to display the highest mutation frequency, nor in the other analysed tissues. In family 8, all the genotyping assays did not detect mosaicism in the phenocopy or her healthy parents, supporting the hypothesis of a de novo occurrence of the BRCA2 mutation identified in the proband. These results suggest that somatic mosaicism is not likely to be a common phenomenon in BRCA1/2 families. As our families fulfilled high-risk selection criteria, other genetic factors might be responsible for most of these cases and have a significant impact on risk assessment in BRCA1/2 families. Finally, we found a de novo BRCA2 mutation, suggesting that, although rare, this event should be taken into account in the evaluation of high-risk families.

Fig 1. Mass spectra of phenocopies (white arrows) and mutation carriers (black arrows) from families 1–8.

Mutant (mut) and wild type (wt) alleles are discriminated by the difference in size. Mass spectra of patients a-g (Families 1–7) refer to DNA extracted from breast tumour; for patients h-i (Families 7–8) the results shown refer to the analysis on DNA extracted from urine samples. The analysis of the father (♂) and mother (♀) of patient 8-i was carried out on blood DNA.

Fig 2. Pedigree of family 8.

The white arrow indicates the phenocopy 8-i; the black arrowhead indicates the family proband (i.e. the first tested family member), carrier of the de novo BRCA2 mutation c.771_775delTCAAA (p.Asn257Lysfs*17). Genetic analysis results (MUT, mutation carrier; WT, wild-type) are reported beneath tested individuals; neoplastic diseases and age at diagnosis in years (y) are reported beneath affected individuals. BrCa, breast cancer; CNS, central nervous system;?, uncertain data.